Drawn arc fastener or stud welding generally includes the shorting of a stud or fastener to the work piece followed by passing a small pilot current into the short circuit followed by lifting of the stud or fastener from the work piece and drawing a pilot arc plasma. Next, the arc current is increased to a higher main arc current melting the weld end of the fastener and the work piece followed by plunging of the fastener into the work piece forming a weld.
The arc energy or heat input delivered by such a process must match with the stud diameter and base metal thickness and must take into account the heat sinking properties of the materials. Too little or too great of a heat input may cause excessive or insufficient melting forming a poor weld. Drawn arc welding typically requires a ceramic ferrule about the stud weld base to weld fasteners on plate material and is generally a fast process done in less than 1 second which may result in a very fast cooling rate. Short cycle or short time stud welding process typically does not require a ferrule and is done in less than 100 ms typically for welding fasteners on thin sheet metals. In some applications, a desired or optimum heat suitable for melting may not be suitable for weld metallurgy. For example, conventional drawn-arc stud welding on MIL-A 46100 armor plate with 0.64 carbon equivalent and NORSOK M-120 offshore structural plate with 0.42 carbon equivalent without preheat will result in brittle microstructure and poor ductility. In another example, drawn arc welding a ⅝″ diameter carbon steel fastener to a lean duplex stainless steel workpiece in 700 millisecond may cause martensitic microstructure in the weld causing a brittle weld that may be prone to failure. Preheating or slower welding processes such as SMAW or GMAW process are known and can reduce the cooling rate but at a longer fabrication time and larger labor cost.
There is therefore a need in the art for a cost-effective method of slowing down the cooling rate to achieve a more ductile or desired microstructure.
Efforts to slow down a cooling rate include increasing a heat input through increasing the main arc current and time. However, excessive current and time may cause process instability and result in excessive stud or fastener burn off, base metal melt through, and lateral expulsion. When utilizing such a technique, molten liquid accumulates at the weld end of the fastener or stud during welding and requires an extreme lift to avoid the accumulated liquid at the stud end from bridging to the molten puddle in the workpiece thereby extinguishing the arc. However, extreme lift or extreme arc length may result in higher susceptibility to arc blow and uneven melting of the stud.
There is therefore a need in the art for an improved arc welding process that adds a controllability of the process forming a stable welding process in which more heat can be delivered in a controlled manner. There is also a need in the art for a process in which the heat can be delivered slowly over a significantly longer weld time slowing the cooling rate of the drawn arc fastener welding process independent of the total heat input.
Drawn arc fastener welding in an underwater environment and heat sinking capacity result in rapid cooling of the weld and heat affected zone and in a localized quenching of the materials. The quenching may increase the hardness in both the weld and heat affected zone. Such quenching may result in brittleness of the stud weld. Additionally wet welding has excessive hydrogen dissolved in the weld pool and fast welding time and rapid freezing can trap the hydrogen bubbles causing increased porosity in a weld. There is therefore a need in the art for a process that may be utilized in underwater welding to increase a weld time and slow down a cooling rate to lower the hardness and porosity of a formed weld.
Drawn arc fastener welding may be performed on a coated work piece such as a galvanized beam or zinc coated work piece or a fastener that may be zinc coated or galvanized. Zinc may be utilized for corrosion resistance but has a detrimental effect on a weld. Generally, to avoid weld contamination the zinc must be removed from the galvanized work piece prior to a welding by grinding or other such labor-intensive methods. Additionally, fasteners or studs that include a zinc coating must be manufactured such that the weld end of the fastener does not include the zinc coating, adding to an overall cost of the stud. Generally, zinc has a low boiling temperature and may be vaporized if sufficient time is provided in a heated condition. However, conventional or typical drawn arc welding has a time that is too fast to vaporize the zinc material in the weld zone. There is therefore a need in the art for an improved process that increases a weld time and maintains a stable welding process that vaporizes surface zinc and other contaminations such as surface oxides and moisture.
Generally, drawn arc fastener welding for larger diameter fasteners requires a very high output power source such as greater than 1500 or 2000 amps. Larger power sources require higher current service line powers and larger generators. There is therefore a need in the art for a process that utilizes a smaller power source and utilizes a fraction of the welding current with an increase of the time for the weld to deliver an equivalent heat input for melting a larger fastener.